Monica, I cannot speak for those who give cash (or any gift) on Yom Tov as this is considered a business transaction and is rabbinically forbidden as a safeguard of the labor of writing. But there is more leeway to talk about the transaction on Yom tov.
Transferring an object to another's possession is definitely a violation of business activities. Talking about business is not a violation of the above, but is a violation of Yishaya 58:13-14 which warns to steer clear of "your business actions". All would agree, though, that if the business is not personal but spiritual (chafatzecha excludes cheftzei shamayim), one would be permitted to talk about business. For example, one would be allowed to tell a mohel that he is interested in hiring him for a job.
There is a debate over how much one can talk about (Rema OC 306:3 and 6. Can I actually set a price with the mohel and strike a deal. Some say this is still permitted since it is spiritual business. Others say this is too close to the rabbinic decree of business activities (similar to transfering objects) which were forbidden under all circumstances (MB 306:14). [The Mishna Berurah 306:32 equates the debate in 306:3 to 306:6 as well.]
Interestingly, whereas in 306:3 the Rema sides with the stricter approach to forbid making the deal ("v'chen ikar"), he says in 306:6 that the custom is to be lenient and make specific pledges to charity or to pay the chazan ("v'haminhag l'hakil").
So, since you are setting a price for a spiritual deal to eat an afikomen (that you can just take another out of the box does not undermine the spiritual aspect of the "stolen" matza), and you are not transferring the possession since it's already yours, you are at least in line with the "custom".